A two-terminal or four-terminal measuring apparatus has been widely utilized to measure the impedance of circuit elements such as a resistor, an inductor, a capacitor, etc. FIG. 1 shows one example of the two-terminal-connection-type measuring apparatus. In this example, both terminals of a device under test (DUT) 1 are respectively connected to measuring terminals H and L. A measuring voltage is applied to the measuring terminals H and L by a voltage source 11 (V.sub.S), and a current flowing through the measuring terminals H and L is measured by an ammeter 12 (A.sub.m). The impedance of the DUT 1 is obtained from the vector ratio of values read by the voltage source 11 and the ammeter 12.
However, when an abnormal contact is made between the DUT 1 and the measuring terminals H and L during an impedance measurement in the above measuring apparatus, an error value is inevitably obtained as a measured impedance value. Although this abnormality may be caused by defect in the DUT 1 itself or by an imperfect contact, it is not easy to determine a cause for the erroneous impedance value. In order to overcome this disadvantage, a method has been proposed based upon an assumption that an impedance value which is measured under the imperfect contact is substantially larger than that under normal contact. According to this method, a predetermined highest value is set as a threshold value, and whether or not a normal contact has taken place is determined with respect to this threshold value. An abnormal contact includes an imperfect contact or non-contact. However, when the above method is applied to a capacitor as the DUT 1 for an insulation test, since the capacitor inherently has large resistance, a proper predetermined highest limit value cannot be easily determined.
In order to overcome the above disadvantage, another method has been proposed in which an electrical property of the DUT is considered as shown in FIG. 2. FIG. 2 is a circuit diagram for the insulation test of the DUT 1 which comprises a capacitor C.sub.X and a resistor R.sub.X. A DC voltage and an alternating voltage are respectively applied to the measuring terminals H and L by a DC voltage source 21 (V.sub.S1) and an alternating voltage source 22 (V.sub.S2). The DC current is measured through a resistor R by a DC ammeter 23 (A.sub.m1), while the alternating current is measured through a capacitor C by an alternating ammeter 24 (A.sub.m2). In the original insulation test, a current is caused to flow through the resistor R.sub.X by the DC voltage source 21, and the current value flowing through the resistance R.sub.X is measured by the DC ammeter 23. If an abnormal contact is made between the DUT 1 and the measuring terminal H or L, the alternating current that flows through the capacitor C.sub.X is very small. Thus, the measured value of the alternating ammeter 24 is distinctly lower than a normal value. Therefore, the contact judgement can be made with high accuracy in the above method.
In the measuring apparatuses as shown in FIGS. 1 and 2, inductance of circuit elements other than the DUT 1 such as a cable in a measuring circuit, is not taken into account. The alternating voltage (V.sub.S, V.sub.S2) is merely regarded as the voltage across the DUT. Thus, the accuracy in the impedance measurement using the above measuring apparatuses is not acceptable. A four-terminal measuring apparatus substantially improves the impedance measurement by overcoming a measurement error due to extraneous inductance, e.g., from a connecting cable.
FIG. 3 shows an example of a prior art four-terminal impedance measuring apparatus with a contact judging circuit. In FIG. 3, one end of the DUT 1 is connected to measuring terminals H.sub.C (current terminal for high voltage) and H.sub.P (voltage terminal for high voltage). The other end of the DUT 1 is connected to measuring terminals L.sub.C (current terminal for low voltage) and I.sub.P (voltage terminal for low voltage). This four-terminal impedance measuring apparatus is designed to selectively switch the connections (41-44) among the DUT 1, plural voltage sources and meters for measuring the impedance and judging the contact. When the switches 41 through 44 are closed on the A side as shown in FIG. 3, a voltage signal from an alternating voltage source 31 (V.sub.S1) is applied to the terminals H.sub.C and L.sub.C, and the voltage across the DUT 1 is measured by an alternating voltmeter 32 (V.sub.m1) connected across the measuring terminals H.sub.P and L.sub.P. At the same time, the current flowing through the DUT 1 is measured by an ammeter 33 (A.sub.m1) which is connected to the measuring terminal L.sub.C. The impedance of the DUT 1 is obtained from the vector ratio between the measured voltage and current values.
Still referring to FIG. 3, when the switches 41 through 44 are closed on the B side (not shown), a voltage signal from a DC voltage source 34 (V.sub.S2) is applied to the measuring terminals H.sub.C and H.sub.P, and a voltage signal from a DC voltage source 36 (V.sub.S 3) is applied to the measuring terminals L.sub.C and L.sub.P. If the contact at each terminal is normal, the voltmeters 35 and 37 respectively read the voltages from voltage sources 34 and 36. However, if any contact is improper, either of the voltmeters 35 and 37 reads a lower value than the expected voltage. Therefore, the contact is easily judged using this prior art apparatus.
In general, the following factors are considered in determining whether the contact judging operation is effective.
(1) The time required for the contact judgment is relatively short. PA1 (2) The construction of the contact judgment apparatus is simple. PA1 (3) The restriction due to the electrical characteristics of a DUT is relatively minor. PA1 (4) The energy delivery to and from a DUT is low.
The first two conditions are satisfied by the four-terminal measuring apparatus as shown in FIG. 3. However, when the DUT comprises a capacitor or inductor, energy is frequently accumulated in the circuit elements, and current flows into the DUT. Therefore, if the conditions (3) and (4) are critical, a switch for timing the application of voltage signals and or a circuit for preventing a DC current flow into the DUT must be added to the prior art apparatus. These countermeasures sacrifice the condition (2) and also cause a new transient phenomenon.
The current invention has solved the above problems by providing a contact judging circuit and method for DUT impedance measuring in which a contact between a DUT and each measuring terminal is accurately judged with a simple circuit.